Re: [Qemu-devel] [PATCH v5 13/17] migration: Create thread infrastructure for multifd recv side

[Juan Quintela <quintela@redhat.com>]

Peter Xu <peterx@redhat.com> wrote:
> On Mon, Jul 17, 2017 at 03:42:34PM +0200, Juan Quintela wrote:

>> +static void multifd_recv_page(uint8_t *address, uint16_t fd_num)
>> +{
>> +    int thread_count;
>> +    MultiFDRecvParams *p;
>> +    static multifd_pages_t pages;
>> +    static bool once;
>> +
>> +    if (!once) {
>> +        multifd_init_group(&pages);
>> +        once = true;
>> +    }
>> +
>> +    pages.iov[pages.num].iov_base = address;
>> +    pages.iov[pages.num].iov_len = TARGET_PAGE_SIZE;
>> +    pages.num++;
>> +
>> +    if (fd_num == UINT16_MAX) {
>
> (so this check is slightly mistery as well if we don't define
>  something... O:-)

It means that we continue sending pages on the same "group".  Will add a
comment.

>
>> +        return;
>> +    }
>> +
>> +    thread_count = migrate_multifd_threads();
>> +    assert(fd_num < thread_count);
>> +    p = multifd_recv_state->params[fd_num];
>> +
>> +    qemu_sem_wait(&p->ready);
>
> Shall we check for p->pages.num == 0 before wait? What if the
> corresponding thread is already finished its old work and ready?

this is a semaphore, not a condition variable.  We only use it with
values 0 and 1.  We only wait if the other thread hasn't done the post,
if it has done the post, the wait don't have to wait. (no, I didn't
invented the semaphore names).

>> +
>> +    qemu_mutex_lock(&p->mutex);
>> +    p->done = false;
>> +    iov_copy(p->pages.iov, pages.num, pages.iov, pages.num, 0,
>> +             iov_size(pages.iov, pages.num));
>
> Question: any reason why we don't use the same for loop in
> multifd-send codes, and just copy the IOVs in that loop? (offset is
> always zero, and we are copying the whole thing after all)

When I found the function, I only remembered to change one of the
loops.  Nice catch.

Later, Juan.